The present invention relates to a method of printing polymer films and corresponding articles. The invention is useful for providing dimensional stability during printing and/or improving the print quality, particularly for contact or thermal printing methods such as thermal mass transfer printing.
There are several problems associated with printing on unsupported thin polymeric films. In general, thin films are less dimensionally stable and lack sufficient rigidity, causing such films to be difficult to handle and feed through a printer. Further, polymer films tend to carry very high static charges that attract dust particles causing void areas in the printed graphic. Thin polymer films also tend to be somewhat uneven in thickness in machine direction as well as in cross-web direction and have microscopic bumps and voids. These factors can cause uneven head pressure between the film and print head resulting in non-uniform colorant transfer.
In the case of thermal printing, and in particular thermal mass transfer printing, thin films typically conduct heat very rapidly, thus requiring higher levels of heat energy to effectively transfer the colorant from the ribbon to the receiving substrate. This increased temperature and/or increased contact time between the print head and thin polymer film, in turn can cause film wrinkling as well as reduced print head life. Wrinkling creates print voids in the creases of the wrinkles as well as misalignment of the film as it travels past the print head. The problems associated with heat-induced stresses are more pronounced in wider printers and printers with multiple heads.
Due to these aforementioned problems, unsupported thin polymer films often exhibit poor print quality when imaged by various printing techniques and in particular thermal printing methods as well as printing methods that involve contact between the image-receiving substrate and the print device such as in the case of thermal mass transfer printing.
Thin polymeric films are often used as top films in the construction of various commercial graphics films as well as various retroreflective sheeting for signage and other uses. In view of the problems associated with printing unsupported films, one approach is to print a dimensionally stable substrate, such as a thick polymeric film, optionally comprising an ink receptive layer. The topfilm may be mirror image printed and then bonded, typically by means of a permanent grade adhesive to a second substrate (e.g. retroreflective substrate) such that the printed surface layer is buried between the topfilm and the substrate. Alternatively, a dimensionally stable substrate may be image directly. A transparent topfilm or topcoat may then be applied to the viewing surface to protect the exposed print from environmental degradation. Another approach is to provide a construction (e.g. commercial graphic film) that comprises a thin polymeric film having a printable surface, optionally comprising an ink receptive layer. A pressure sensitive adhesive (PSA), covered with a release liner is provided on the surface of the film opposing the printable surface, resulting in a laminate having the PSA sandwiched between the film and the release liner. The exposed surface of the thin polymeric film of the laminate is then printed. During use the release liner is removed such that the pressure sensitive adhesive cleanly separates from the release liner, the adhesive remaining on the non-viewing surface of the printed film. The adhesive coated surface is then contacted to the target surface, such as a billboard backing.